1. Field of the Invention
The present invention relates to a method for assembling a drum drive unit which drives photoreceptor drums and an image formation apparatus having the drum drive unit, and in particular, to a technology of synchronizing rotational speeds of a plurality of photoreceptor drums.
2. Description of the Related Art
An image formation apparatus carrying out image formation by use of an electrophotographic system has been known.
In such an image formation apparatus, an electrostatic latent image is formed on a surface of a photoreceptor drum, and a toner image is formed by making toner adhere to the electrostatic latent image by a developing machine.
Next, the toner image is transcribed onto a recording medium such as paper, an OHP, or the like, and the toner image transcribed on the recording medium is fixed with a fixing machine. Among such image formation apparatuses, there is a tandem type image formation apparatus which uses a plurality of photoreceptor drums, and forms a color image by sequentially superimposing respective color toner images formed on the respective photoreceptor drums, on an intermediate transcriptional body.
In such a tandem type image formation apparatus, it is necessary for the respective photoreceptor drums to rotate at a constant rotational speed and at an equivalent angular velocity. Namely, when the respective photoreceptor drums are uneven in rotational speeds, there occurs the problem that misalignment in the toner images is brought about depending on respective colors at the time of transcribing toner images onto an intermediate transcriptional body, which makes it impossible to carry out satisfactory image formation.
However, the respective photoreceptor drums respectively fluctuate in rotational speeds with a cycle. As factors of the rotational speed fluctuations of the photoreceptor drums, there can be quoted mesh errors among driving force transmission members which are on the shafts of the photoreceptor drums, and transmit a rotation to the shafts of photoreceptor drums, and the like.
The mesh errors will be described. In driving force transmission members prepared by injection molding, mesh errors as shown in FIG. 4 are brought about in consequence of a structure of a metallic mold in the manufacturing process. FIG. 4 is a graph in which mesh errors of gears are measured by a mesh tester. When an attempt is made to transmit a driving force by meshing a driving force transmission member in which a mesh error is brought about with a driving force transmission member in which there is no mesh error, rotational speed fluctuations at a constant cycle are brought about depending on an extent of the meshing between the driving force transmission members.
Due to the driving force transmission members rotary-driven with rotational speed fluctuations in a constant cycle, the photoreceptor drums to which a driving force is coaxially transmitted from the driving force transmission members have the same rotational speed fluctuations as those in a constant cycle of the driving force transmission members under the influence of the rotational speed fluctuations of the driving force transmission members. Therefore, at the time of transcribing respective color toner images formed on the photoreceptor drums, onto a non-transcriptional body such as an intermediate transcriptional body or the like, misalignment in the toner images is brought about depending on the respective colors, which brings about color shift.
As means for solving this problem, there can be employed a method in which the precision in molding the driving force transmission members is improved, which eliminates mesh errors of the driving force transmission members. However, the precision can be raised only to a certain extent in consideration of mass productivity or the like. Then, conventionally, there has been a tandem type image formation apparatus which has a driving motor rotary-driving photoreceptor drums, and in which rotational speeds of the photoreceptor drums are synchronized due to the respective photoreceptor drums independently controlling the mechanism that detects rotational speeds of the photoreceptor drums by use of means such as a photosensitive element or the like, and adjusts the speeds by the driving motor (Jpn. Pat. Appln. KOKAI Publication No. 2002-311672).
In accordance with the above-described conventional apparatus, two rotational speed fluctuations of the driving motor rotating the photoreceptor drums are read by use of an encoder, and the rotational speed fluctuations are reduced by feedback-controlling the driving motor.
Further, comparison with rotational speed fluctuations of other photoreceptor drums is carried out, and rotational speed fluctuations are synchronized. In such an apparatus, it is necessary to provide a sensor detecting a rotational speed of a photoreceptor drum, and a driving motor for adjusting a rotational speed of each photoreceptor drum, to each of the respective photoreceptor drums. Therefore, a number of components increases, which brings about a high cost.